Small Craft Advisory 284, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
400 PM AKDT Wed Apr 4 2018

.ANALYSIS AND UPPER LEVELS...
A broad upper level low centered over the Kuskokwim Delta continues
to be the primary weather feature impacting the Southern Mainland.
A strong shortwave rotating along the eastern edge of the low is
beginning to bring widespread snow shower activity to much of the
North Gulf Coast as well as areas of the Kenai Peninsula. Over the
Anchorage Bowl however very dry low levels are preventing any of
these showers from reaching the surface, though areas of virga are
beginning to appear as these showers begin to move off of higher
elevation. A weak surface trough that developed along the Northern
Gulf yesterday evening strengthened to gale force early this
morning near the entrance to Prince William Sound and the Barren
Islands. These winds are all beginning to diminish however as the
trough begins to make landfall over the Kenai Peninsula and
dissipates.

Further west over the Southwest Mainland the weather is much
quieter with only some snow shower activity under the upper low in
Bristol Bay to speak of. The Bering Sea and Aleutians regions are
in a fairly benign weather pattern with widespread northeasterly
flow and cold air advection.

&&

.MODEL DISCUSSION...
The models remain in very good synoptic agreement through the
short term with the slow progression of the retrograding low
through the southern Mainland. There was some disagreement in the
overnight models with the placement of the heaviest snowfall that
is expected to develop over the Southwest Mainland on Thursday,
however agreement began to improve significantly with today's 12z
runs. This has led to increasing confidence in the snow forecast,
especially over Bristol Bay. Good model agreement then continues
with the next incoming Western Aleutian low late Thursday into
Friday, leading to generally good forecast confidence for most
areas into the end of the work week.

&&

.AVIATION...
PANC...There will be two forecast concerns for the terminal this
evening. The first will be with determining how low conditions
fall as snow showers move through the area this evening. So far
dry air in the low levels has prevented much of these showers from
reaching the surface, however as heavier showers move through in
advance of a shortwave, the low levels are still expected to
moisten enough to bring MVFR to possibly even brief IFR conditions
this area. The potential for LLWS will then move in as strong SE
winds aloft develop, ending the potential for significant snow
showers. This will likely bring gusty SE winds to the terminal
for much of the overnight hours, with enough wind aloft to
produce LLWS. Both the SE winds and LLWS should then diminish
Thursday morning as the down inlet flow increases again.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA...
Heavy snow looks to continue through the evening for Prince
William Sound before tapering down into Thursday morning as
showers keep pouring in from the east. Heaviest snowfall will be
for the eastern Kenai Peninsula. Additional snow accumulations
of 6-12 inches are expected for the eastern Kenai Peninsula
overnight with the highest amounts near the coast and at higher
elevations.

Snow showers have also made it over the coastal mountains and have
saturated the dry air around Cook Inlet. However, the easterly
flow will cause downslope drying that will keep any accumulations
to a minimum for places around Anchorage. The uncertainty comes
with the possibility of localized showers that could bring heavier
snow to localized areas for a short time. Higher elevations
around Anchorage will be an exception, being out of the dry air,
and look to see a couple inches of snowfall overnight.

The weather will then clear on Thursday with weak high pressure
building in from the east and afternoon temperatures rising to
around 40F. This will last until Friday afternoon when a weak wave
from the south will bring a mix of light precip to the north Gulf
coast.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2: Thursday and
Friday)...
Weak upslope snow showers are ongoing across Dillingham and points
north along the Wood-Tikchik Mountains this afternoon. These snow
showers will likely continue through the region overnight, though
should shift west of Dillingham later this evening. Snow showers
are expected to break out across much of the remainder of Bristol
Bay and the Lower Kuskokwim Valley late tonight as an upper level
disturbance pinwheeling around the upper level low south of
Kuskokwim Bay increases the atmospheric lift through the area
while at the same time the flow becomes increasingly south to
southwesterly, bringing in more moisture.

The snow showers that develop early Thursday are expected to
consolidate into a single band through the morning. The main
question and single greatest point of uncertainty in the short-
term forecast is where the band will set up. The models have
generally been shifting westward with this feature over time, but
unfortunately a clear answer may not really be known until it's
actually formed. Right now our best forecast is that it will set
up near or just east of King Salmon, in essentially a north-south
band that will encompass Igiugig, New Stuyahok, and continue
northward through Sleetmute. Instability will be large enough that
the affected communities will likely see large changes in
visibility as individual embedded snow showers move through. There
may also be large variability in snowfall amounts as well, and
only a few miles may be the difference between less than an inch
of snow to as much as 6 inches where the snow band persists
longest. Since temperatures will be warm enough to keep snow
ratios down, and the expected isolated nature of the greatest
snowfall amounts, no advisory is thought needed at this time. A
few hit-and-miss snow showers are possible through the Kuskokwim
Delta on Thursday, but the main action as far as precipitation
goes will be further east.

On Thursday night, the snow band will diminish in intensity and
fall apart during the evening, though scattered snow showers will
continue through the night into Friday, as they generally drift
back eastward towards the foothills of the Alaska Range where
uplift will be aided by the topography. Elsewhere, Friday into the
weekend looks to be quiet area-wide.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2: Thursday
and Friday)...
Ocean-effect snow showers ongoing through the northeastern half
of the Bering will continue through into Thursday as cold air
moving over the relatively warmer water picks up moisture. Over
the western Bering and Aleutians, fog is the primary weather
hazard. This is associated with a front that too is picking up
moisture from off the Bering, but unlike further east, a warm air
mass above it is trapping the moisture in the lowest levels of the
atmosphere, resulting in fog. A second much stronger front will
move into the western Aleutians Thursday morning, ending the fog
just in time for more steady rain to move in. The front will not
make much northward progress since steady northeast winds in the
northern Bering will impede its progress, but the front will have
an easier time spreading eastward across the Aleutians and far
southern Bering Thursday night into Friday. So all of these areas
can expect gusty winds with steady rains. On Friday afternoon, the
rain or a rain/snow mix will reach the Pribilofs.

&&

.LONG TERM FORECAST (Days 4 through 7)...
The forecast period commences with a decaying upper level low in
the Gulf Of Alaska and a second low near the Central Aleutians.
The GFS and the ECMWF are in decent agreement with the placement
of both lows but the ECMWF is deeper with the surface low pressure
for both lows. There is a third low south of Kamchatka that will
move towards Attu Island. By Monday however, the divergence in
solutions gradually increases which impacts confidence levels as
to where the gale force winds will materialize. By Tuesday, the
GFS brings a fourth low into the Western Aleutians while the ECMWF
just brings a wave. These differences get compounded with time.
It is a progressive and every global model depicts a train of lows
from Hokkaido to the Central Aleutians. But given the vagaries of
the models, stay tuned for updates on these systems.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Heavy Freezing Spray Warning 180 181.

&&

SYNOPSIS AND MODEL DISCUSSION...DEK
SOUTHCENTRAL ALASKA...DK
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JW
LONG TERM...PS

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE ANCHORAGE AK
946 PM AKDT WED APR 4 2018

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0900 PM     HEAVY SNOW       6 NNE SEWARD            60.19N 149.38W
04/04/2018  E11.0 INCH       KENAI PENINSULA    AK   PUBLIC

            MILE 6.5 SEWARD HIGHWAY (AT QUESTA WOODS).

&&

SEB

SPECIAL WEATHER STATEMENT...UPDATED
National Weather Service ANCHORAGE AK
340 AM AKDT Thu Apr 5 2018

AKZ101-121-125-051245-
Anchorage-Western Kenai Peninsula-Western Prince William Sound-
Including the cities of Anchorage, Eagle River, Indian, Eklutna,
Kenai, Soldotna, Homer, Cooper Landing, Whittier, Seward,
Girdwood, and Moose Pass
340 AM AKDT Thu Apr 5 2018

...Snow returning to the Southern Mainland through Thursday...

After a long period of sunny and dry conditions throughout much
of the Southern Mainland, snow is expected to return to the area
beginning this evening. The highest confidence and most
significant snowfall is expected over Western Prince William
Sound, including areas near sea level such as Whittier, Portage,
and Seward that will remain as snow throughout the event. Snowfall
accumulations will be highest from Whittier to Turnagain Pass
where up to a foot of snow is expected, with over 20 inches
possible in the Portage Valley. This event is expected to persist
into Thursday morning.

Confidence in accumulating snowfall over the Western Kenai
Peninsula and Anchorage Bowl is however much less from this event
due to the recent very dry weather pattern. At this point it
doesn't appear that these areas will see significant
accumulations, however the best chance for snow is expected late
Wednesday afternoon and evening, potentially impacting the
afternoon commute.

Snowfall is then expected to begin to diminish throughout the area
Thursday morning into the afternoon.

For the latest forecast for your location, please visit
weather.gov/anchorage.